The anisotropic compression of the crystal structure of 3-aza-bicyclo(3.3.1)nonane-2,4-dione to 7.1 GPa

The crystal structure of 3-aza-bicyclo(3.3.1)nonane-2,4-dione has been determined at room temperature between ambient pressure and 7.1 GPa. The structure consists of chains formed by NH⋯O hydrogen bonds, which are then connected through CH⋯O contacts to form sheets. CH⋯O interactions also connect pairs of sheets into slabs. The dicarboximide moiety of the molecule becomes nonplanar at elevated pressure. This crystal structure has been regarded as anomalous because it fails to conform to expectations regarding a preference for packing based on dimers. However, no phase transition to a dimer-based structure was identified in this study. Analysis of the Hirshfeld surfaces identified the largest intermolecular voids at ambient conditions, and the distribution of these voids is consistent with the direction of largest linear strain developed on compression. The surfaces also facilitated identification of the short contacts which appeared at pressure, including two close CH⋯O interactions and three contacts between alkyl groups. PIXEL analysis shows that the hydrogen bond is relatively weak (14.6 kJ mol ^-1) and that a pair of CH⋯O contacts and a van der Waals interaction are of comparable energy (between 8.7 and 6.0 kJ mol ^-1). The energies of the CH⋯O interactions and various H⋯H contacts in the structure are more affected by pressure than the hydrogen bond, but it appears that the response to pressure is dominated by dispersion interactions rather than hydrogen bonds. This study illustrates the combined use of Hirshfeld surfaces and the PIXEL method is a particularly effective combination for analyzing changes in crystal structures.